1. Field of the Invention
The present invention relates to a method of manufacturing a nitride semiconductor device for use in a laser diode.
2. Description of the Background Art
Because a nitride semiconductor has a large bandgap, gallium nitride (GaN) has particularly been used for a laser diode (LD). In the LD of GaN, an n-type electrode is formed on the back surface of a GaN substrate. In this case, it is necessary to make the substrate thin prior to the formation of the n-type electrode, and the GaN substrate is polished. This polishing, however, causes an altered layer to be formed on a polished surface of the substrate to increase a contact resistance with the n-type electrode to be formed on this surface. A method of reducing this contact resistance includes a method of performing heat treatment at a high temperature of about 700° C. after the n-type electrode is formed. The processing at such a high temperature, however, presents a problem such that a p-type electrode provided on the GaN substrate is degraded.
A solution to such a problem is a method of dry etching the polished surface using a halogen gas as an etching gas after the polishing of the GaN substrate to remove the altered layer generated on the polished surface, and performing heat treatment at a temperature of not less than 400° C. after electrode formation to reduce the contact resistance with the n-type electrode. (See, for example, Japanese Patent Application Laid-Open No. 2003-347660 (pp. 4-5)).
As the LD advances toward ever smaller size and higher power, power inputted to the LD increases, and a further reduction in the contact resistance of the electrode is desired. The method of dry etching the polished surface using the halogen gas to remove the altered layer still has problems in that there is a limit to the reduction in the contact resistance of the n-type electrode and in that the heat treatment at the high temperature of not less than 400° C. is required to reduce the contact resistance.